Powderous solid electrolyte compound for solid-state rechargeable lithium ion battery

ABSTRACT

A solid solution electrolyte suitable for solid-state rechargeable lithium ion battery comprising a compound having a general formula L i(3.5+L+x) Si (0.5+s−x) P (0.5+p−x) Ge 2x O 4+a  wherein −0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.0&lt;x≤0.30, preferably 0.05≤x≤0.30.

TECHNICAL FIELD AND BACKGROUND

This invention relates to a solid electrolyte (SE) for solid-staterechargeable lithium-ion batteries suitable for electric vehicle (EV)applications. The solid electrolyte according to the invention has animproved lithium ionic conductivity.

Along with the developments of EVs, it comes also a demand forlithium-ion batteries as a possible constant source of power for suchapplications.

Aside the zero-emission aspect, additional requirements rendering abattery eligible for EV applications should therefore include: highcapacity, longer cycle lives, lower cost, and better safety.

Most of the past and ongoing research on batteries usually emphasizes onthe use of a liquid electrolyte, since such an electrolyte has a highlithium ionic conductivity of around 10⁻² S/cm at room temperature(measured for an electrolyte composed of ethylene carbonate/dimethylcarbonate EC/DMC 1M LiPF₆) and provides a good contact with electrodesthrough wetting. However, batteries containing liquid electrolytes implyconcerns since liquid electrolytes are usually flammable.

Solid electrolyte-based batteries are considerably safer and can be analternative to liquid electrolyte-based batteries for the power sourceof electric vehicles. Since the decomposition temperature of SE ishigher than the liquid electrolyte, an EV comprising such a SE-basedbattery, instead of a liquid electrolyte-based battery, would be saferand its battery manufacturing would also be safer. Aside from the clearadvantage that a SE-based battery is much safer than a liquidelectrolyte-based battery, a SE-based battery is also more compact thanliquid electrolyte-based batteries, and therefore lead to higher powerdensities

However, it is also true that, compared to liquid electrolyte, a SE hasa lower lithium ionic conductivity (typically included between 10⁻⁷ to10⁻⁶ S/cm). The lower lithium ionic conductivity in a SE is due to thehigher migration energy of lithium ions.

Well-known (inorganic) types of SE suitable for solid-state rechargeablelithium-ion batteries (SSB) which have been mainly explored are the La,Zr comprising garnet and the LSPO (lithium silicon phosphorus oxide)LISICON (lithium superionic conductor) electrolyte having a generalformula: Li_(4±x)Si_(1−x)P_(x)O₄. The La, Zr comprising garnet,especially the cubic garnet (Li₇La₃Zr₂O₁₂), shows a high conductivity.However, those materials are expensive due to the La and Zr content.This compound also needs to be synthesized at a temperature higher than1000° C. which is not desired because of the volatility of lithium atthe high temperature. The LSPO is well-known for its high chemical andelectrochemical stability, high mechanical strength, and highelectrochemical oxidation voltage, making such an electrolyte apromising candidate for EV applications.

LSPO compounds, like Li_(3.5)Si_(0.5)P_(0.5)O₄, have a relatively lowlithium ionic conductivity. Li_(3.5)Si_(0.5)P_(0.5)O₄, is a solidsolution of Li₄SiO₄ and Li₃PO₄ having a crystal structure of γ-Li₃PO₄with orthorhombic unit cell and tetrahedrally coordinated cations. Inthe scope of the present invention, a solid solution (also calledsolid-state solution) refers to a multi-component solid-state solutionas a result of a mixture of one or more solutes in a solvent. Inparticular, the solutes can be atoms or groups of atoms (or compounds).Such a multi-component system is considered a solution rather than acompound when the crystal structure of the solvent remains unchanged byaddition of the solutes, and when the chemical components remain in asingle homogeneous phase. In the system, the solvent usually is acomponent with the largest portion and in this case is Li₃PO₄. Thecrystal structure of the solvent component is maintained after blendingwhereas the other component (solute, e.g. Li₄SiO₄) dissolves in thesolvent structure instead of forming a distinct compound having astructure that deviates from the structure of the solvent.

In Solid State Ionics (2015), 283, 109-114, Wang, Dawei et al. studiedthe enhancement of lithium ionic conductivity of LSPO by an addition ofLi₃BO₃. The highest lithium ionic conductivity was 6.5×10⁻⁶ S/cm at 20%of Li₃BO₃ addition compared to 3.6×10⁻⁶ S/cm for 0% addition. Anotherstudy conducted by Choi, Ji-won et al. in Solid State Ionics (2016),289, 173-179, explored the effect of Al cation substitution in0.7Li₄SiO₄+0.3Li₃PO₄ solid solution system. The lithium ionicconductivity of 7.7×10⁻⁶ S/cm was achieved at room temperature by theaddition of 10 mol % Al.

Sokseiha et al. in Chem. Mater. (2018), 30, 5573-5582 and Kamphorst et.al. in Solid State Ionics (1980), 1, 187-197 specify that LÍ4SÍO4-U3PO3solid solutions have a lower conductivity than Li₄GeO₄—Li₃PO₃ teachingaway from a possible substitution of Ge with Si.

Burmakin et. al. in Russian Journal of Electrochemistry (2010), 46, No.2, 243-246 discloses a lithium germanium phosphate solid electrolytedoped with a tetravalent cation, Zr like:Li_(3.75)Ge_(0.70)Zr_(0.05)P_(0.25)O₄,Li_(3.50)Zn_(0.125)Ge_(0.75)P_(0.25)O₄, respectively. The Ge content inthese formulation would not allow the obtention of a solid solution in aLSPO-based SE.

Although noticeable, these attempts to achieve a solid solution of aLSPO-based electrolyte are still below a desired threshold of at least0.50×10⁻⁵ (or 5.0×10⁻⁶) S/cm required so that a solid-state battery madefrom said electrolyte is a tangible alternative to current liquidelectrolyte-based batteries.

Certainly, there is a need for improving the lithium ionic conductivityof LSPO-based electrolyte so as to render the use of solid-statebatteries made from such an electrolyte more performant and thereforemore attractive in the field of EV applications.

It is therefore an object of the present invention to provide aLSPO-based SE having an improved lithium ionic conductivity whilstretaining a solid solution, which is a prerequisite for the use of sucha SE in a solid-state secondary battery suitable for EV applications.

Metallic Li can be used in the scope of the present invention as ananode of a SSB comprising the electrolyte according to the invention.

Provided that the SE according to the present invention can be destinedto be contacted to a Li metal-based anode, it must therefore becompatible to said Li metal-based anode of the SSB, meaning that the SEwhile contacting said anode must remain chemically stable.

SUMMARY OF THE INVENTION

A solid electrolyte having an improved lithium ionic conductivity whilstretaining a solid solution and being chemically and thermally stablewhile contacting a Li metal anode is achieved by providing a solidsolution according to claim 1 which comprises a LSPO-based electrolytecomprising germanium (Ge) up to 60 mol %. A Ge doped LSPO is referencedhereunder as “LSPGO”.

If a LSPGO-based electrolyte comprises Ge of superior to 60 mol %, itcomprises at least one impurity phase (i.e. Li₂SiO₃), and such aLi₂SiO₃-bearing LSPGO-based electrolyte is not a solid solutionaccording to the present invention. A similar impurity phase is observedin Burmakin et. al. (Li₂ZrO₃ in this case) for a high content of Ge.

In the framework of the present invention, it has been observed that byintegrating up to 60 mol % of Ge in a solid solution of LSPO-basedelectrolyte it was possible to preserve said solid solution propertytogether with a noticeable improvement of the lithium ionic conductionto a minimal value of 0.50×10⁻⁵ S/cm, which constitutes a significantcontribution to what is currently known from the prior art.

It has also been demonstrated that the electrolyte according to thepresent invention is stable in presence of a Li metal foil, confirmingits suitability in a SSB wherein a Li metal foil is used as an anode.

The present invention concerns the following embodiments:

1. —A solid solution electrolyte suitable for solid-state rechargeablelithium ion battery comprising a compound having a general formulaLi_((3.5+L+x))Si_((0.5+s−x))P_((0.5+p−x))Ge_(2x)O_(4+a) wherein−0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.00<x≤0.30.

In a first aspect of the embodiment 1, 0.05≤x≤0.30, preferably0.10≤x≤0.30.

In a second aspect of the embodiment 1, 0.15≤x≤0.30, preferably0.20≤x≤0.30.

2. —The solid solution electrolyte according to embodiment 1 whereinsaid compound has a general formula:Li_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O₄.

3. —The solid solution electrolyte according to the embodiment 1 or 2,having a lithium ionic conductivity of at least 10⁻⁵ S/cm at 25° C.

4. —The solid solution electrolyte according to any of the precedingembodiments, wherein 0.25≤x≤0.30.

5. —The solid solution electrolyte according to any of the precedingembodiments, having a lithium ionic conductivity measured at 25° C.superior or equal to 2.0×10⁻⁵ S/cm and inferior or equal to 3.0×10⁻⁵S/cm, preferably superior or equal to 2.5×10⁻⁵ S/cm and inferior orequal to 3.0×10⁻⁵ S/cm.

6. —The solid solution electrolyte according to any of the precedingembodiments, comprising a crystal structure having an XRD patternmeasured at 25° C. and at a wavelength of 1.5418 Å comprising a firstpeak having a first intensity and a second peak having a secondintensity, said first and second peaks being present in a range of 2θsuperior or equal to 27.5 and inferior or to 30.0±0.5°, said XRD patternbeing furthermore free of peaks at 2θ>37.0±0.5° having an intensitysuperior to said first or second intensity.

7. —The solid solution electrolyte according to any of the precedingembodiments, comprising a crystal structure having an XRD patternmeasured at 25° C. and at a wavelength of 1.5418 Å comprising a firstpeak having a first intensity and a second peak having a secondintensity, said first and second peaks being present in a first range of2θ superior or equal to 27.5 and inferior or to 30.0±0.5°, wherein in asecond range of 2θ superior or equal to 21.0 and inferior or to25.0±0.5° said XRD pattern has no more than three additional peaks, eachof said three additional peaks having an intensity superior to saidfirst or second intensity.

8. —The solid solution electrolyte according to any of the precedingembodiments, comprising a crystal structure having an XRD patternmeasured at 25° C. and at a wavelength of 1.5418 Å comprising a firstpeak having a first intensity and a second peak having a secondintensity, said first and second peaks being present in a first range of2θ superior or equal to 27.5 and inferior or to 30.0±0.5°, wherein in asecond range of 2θ superior or equal to 34.0 and inferior or to36.0±0.5°, said XRD pattern has no more than three additional peaks,each of said three additional peaks having an intensity superior to saidfirst or second intensity.

9. —A solid-state rechargeable lithium ion battery comprising the solidsolution electrolyte according to any of the previous embodiments.

10. —A solid-state rechargeable lithium ion battery comprising anegative electrode having a Li metal-base anode contacting the solidsolution electrolyte according to any of the embodiments 1 to 9.

11. —Use of the solid-state battery according to the embodiment 9 or 10in an electric vehicle, in particular in an electric car, wherein theoperating voltage of said battery is superior or equal to 200 V andinferior or equal to 500 V.

12. —A catholyte comprising the solid solution electrolyte according toany of the preceding embodiments and a cathode material having thegeneral formula: Li_(1+k)M′_(1−k)O₂ whereM′=Ni_(1−x′−y′−z′)Mn_(x′)Co_(y′)A_(z′) with −0.05≤k≤0.05, 0≤x′≤0.40,0.05≤y′≤0.40, and 0≤z′≤0.05, wherein A is a doping element which isdifferent to Li, M′ and O, said positive active material powdercomprising particles having a layered R-3m crystal structure, saidcatholyte having a D99≤50 μm and an ionic conductivity of at least1.0×10⁻⁶ S/m

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Comparison of Electrochemical Impedance Spectra (EIS) for samplewith various Ge dopants (x inLi_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O_(4+a)) in Example 1 andComparative Example 1

FIG. 2. Variation of lithium ionic conductivity versus the amount of Gedopants (x in Li_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O_(4+a)) inExample 1 and Comparative Example 1 FIG. 3. Comparison of X-raydiffraction pattern of Example 1 and Comparative Example 1

FIG. 4. Fourier-transform infrared spectrum of EX1-A, EX1-B, EX1-C, andComparative Example 1 showing transmittance in (%) and wavenumber in(cm⁻¹)

FIG. 5. Comparison of X-ray diffraction pattern of Comparative Examples1 and 2

FIG. 6. Comparison of X-ray diffraction pattern of LSPO (a) beforeexposure to molten Li metal at 250° C. and (b) after 15 minutes exposure

FIG. 7. Comparison of voltage vs. capacity graph of EX2-CAT-A andCEX3-CAT-B

DETAILED DESCRIPTION

In the following detailed description, preferred embodiments aredetailed so as to enable practice of the invention. Although theinvention is described with reference to these specific preferredembodiments, it will be understood that the invention is not limited tothese embodiments. To the contrary, the invention includes numerousalternatives, modifications and equivalents as will become apparent fromconsideration of the following detailed description.

This invention discloses a germanium bearing LSPO compounds having ageneral formula Li_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O_(4+a)(LSPGO) wherein 0.00<x≤0.30 and −0.40≤a≤0.40. It is a solid solution ofLi₄GeO₄, Li₃PO₄, and Li₄SiO₄. When Ge equally substitutes Si and P in aLSPO compound, it is observed that the lithium ionic conductivity of thecompound according to claim 1 significantly increases (reaching at least10⁻⁵ S/cm) for x values higher or equal to 0.10. Moreover, theconductivity of Li_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O_(4+a)wherein 0.10≤x≤0.30 gradually increases and it is maximized when x is0.30. In particular, the conductivity of the compound according to claim1 is unexpectedly higher (higher than 2.0 10⁻⁵ S/cm) for the narrowrange: 0.20<x≤0.30, in particular for 0.25≤x≤0.30. Compared to thebroader range of Ge content of claim 1, this narrower range leads to anincrease of the lithium ionic conductivity by a 1.5 to 3.0 factor, whichis remarkable.

Such a high conductivity of at least 10⁻⁵ S/cm has never been reportedyet for solid solution LSPO type of electrolytes.

In addition, compatibility of the electrolyte according to the presentinvention with a Li metal anode.

This invention is also inclusive of a catholyte compound made from amixture of a NMC type of positive electrode active material and thegermanium bearing LSPO compound according to the present invention.

Both polycrystalline and monolithic NMC can be used as a positiveelectrode active material in the catholyte according to the presentinvention. A “monolithic” morphology refers here to a morphology where asecondary particle contains basically only one primary particle. In theliterature they are also called single crystal material, mono-crystalmaterial, and one-body material. The preferred shape of the primaryparticle could be described as pebble stone shaped. The monolithicmorphology can be achieved by using a high sintering temperature, alonger sintering time, and the use of a higher excess of lithium. A“polycrystalline” morphology refers to a morphology where a secondaryparticle contains more than one primary particles.

The (solid-state) catholyte material is prepared by mixing the germaniumbearing LSPO compound with the NMC composition so as to produce thecatholyte which is subjected to a heat treatment at 600° C.˜800° C. for1˜20 hours under oxidizing atmosphere. In particular, the method forproducing said catholyte is a co-sintering-based process wherein thegermanium bearing LSPO and the NMC compositions are blended so as toprovide a mixture which is then sintered.

There are several ways to obtain each of the compositions of thegermanium bearing LSPO and NMC positive electrode active material in thecatholyte. Whereas the difference of the median particle sizes (D50)between the solid electrolyte and positive electrode active material inthe catholyte is superior or equal to 2 μm, they can be separated usinga classifier such the elbow jet air classifier(https://elcanindustries.com/elbow-jet-air-classifier/). Thecompositions of separate particles are measured according to theprotocol disclosed in the section F) Inductively Coupled Plasma methodso as to determine each of the compositions of the germanium bearingLSPO material and NMC positive electrode active material in thecatholyte.

The use of an Electron Energy Loss Spectroscopy (EELS) in a TransmissionElectron Microscope (TEM) is another example for obtaining each of thecompositions of the germanium bearing LSPO material and NMC positiveelectrode active material in the catholyte. The elements and theiratomic amount can be obtained directly by measuring the EELS ofcross-sectional germanium bearing LSPO particles and NMC positiveelectrode active material particles separately.

The following analysis methods are used in the Examples:

A) Electrochemical Impedance Spectroscopy (EIS)

A cylindrical pellet is prepared by following procedure. 0.175 g of apowderous solid electrolyte compound sample is put on a mold having adiameter of 1.275 cm. A pressure of 230 MPa is applied to the mold. Thepellet is sintered at 700° C. for 3 hours in oxygen atmosphere. Silverpaste is painted on both sides of the pellet to have a sampleconfiguration of Ag/pellet/Ag in order to allow EIS measurements.Standard deviation of this measurement is 2.0×10⁻⁸.

EIS is performed using an Ivium-n-Stat instrument, apotentiostat/galvanostat with an integrated frequency response analyzer.This instrument is common to be used in the battery/fuel cell-testing tocollect impedance response against frequency sweep. The measurementfrequency range is from 10⁶ Hz to 10⁻¹ Hz. The setting point/decade is10 and the setting voltage is 0.05V. Measurement is conducted at roomtemperature (at 25° C.). The lithium ionic conductivity is calculated bybelow equation:

$\sigma = \frac{L}{R \times A}$

where L is the thickness of the pellet, A is the area of the sample, andR is the resistance obtained by the electrochemical impedancespectroscopy.

B) X-Ray Diffraction Test

A cylindrical pellet is prepared by following procedure. 0.175 g of apowderous solid electrolyte compound sample is put on a mold having adiameter of 1.275 cm. A pressure of 230 MPa is applied to the mold. Thepellet is sintered at 700° C. for 3 hours in oxygen atmosphere.

The X-ray diffraction pattern of the pellet sample is collected with aRigaku X-Ray Diffractometer (D/MAX-2500/PC) using a Cu Kα radiationsource emitting at a wavelength of 1.5418 Å. The instrumentconfiguration is set at: 1° Soller slit (SS), 1° divergence slit (DS)and 0.15 mm reception slit (RS). Diffraction patterns are obtained inthe range of 10-70° (2θ) with a scan speed of 4° per a minute. ObtainedXRD patterns are analyzed by the Rietveld refinement method using X'PertHighScore Plus software. The software is a powder pattern analysis toolwith reliable Rietveld refinement analysis results.

C) Fourier-Transform Infrared (FTIR) Spectrometry

FTIR transmission spectrum for the LSPGO powder is collected usingThermo Scientific FTIR Spectrometer (Nicolet iS 50) in the wave numberrange of 1200 to 500 cm⁻¹, with a resolution of 4 cm⁻¹, and scan cycleof 32 scan.

D) Particle Size Distribution

The catholyte powder samples used in the particle-size distribution(psd) measurements are prepared by hand grinding the catholyte powdersamples using agate mortar and pestle. The psd is measured by using aMalvern Mastersizer 3000 with Hydro MV wet dispersion accessory afterhaving dispersed each of the catholyte powder samples in an aqueousmedium. In order to improve the dispersion of the catholyte powder,sufficient ultrasonic irradiation and stirring is applied, and anappropriate surfactant is introduced. D50 and D99 are defined as theparticle size at 50% and 99% of the cumulative volume % distributionsobtained from the Malvern Mastersizer 3000 with Hydro MV measurements.

E) Coin Cell Test

For the preparation of a positive electrode, a catholyte containing 0.16g of NMC, 0.03 g conductor (Super P), and 0.125 g of 8 wt % PVDF binderare mixed in NMP solvent using a planetary centrifugal mixer (Thinkymixer) for 20 minutes. The homogenized slurry is spread on one side ofan aluminum foil using a doctor blade coater with a 15 μm gap. Theslurry-coated foil is dried and punched as 8 mm diameter circular shape.A Swagelok cell is assembled in an argon-filled glove box with theconfiguration of positive electrode, separator having a diameter of 13mm, and lithium foil having a diameter of 11 mm as a negative electrode.1M LiPF₆ in EC/DMC (1:1 wt %) is used as electrolyte. Each cell iscycled at 25° C. using automatic battery cycler Wonatech-WBCS3000. Thecoin cell testing at 0.1 C in the 4.3˜2.5V/Li metal window range.

F) Inductively Coupled Plasma (ICP)

The composition of a positive electrode active material, a solidelectrolyte, and a catholyte is measured by the inductively coupledplasma (ICP) method using an Agillent ICP 720-ES. 1 gram of a powdersample is dissolved into 50 mL high purity hydrochloric acid (at least37 wt % of HCl with respect to the total amount of solution) in anErlenmeyer flask. The flask is covered by a watch glass and heated on ahot plate at 380° C. until complete dissolution of the powder. Afterbeing cooled to room temperature, the solution from the Erlenmeyer flaskis poured into a 250 mL volumetric flask. Afterwards, the volumetricflask is filled with deionized water up to the 250 mL mark, followed bya complete homogenization. An appropriate amount of solution is takenout by a pipette and transferred into a 250 mL volumetric flask for asecond dilution, where the volumetric flask is filled with internalstandard and 10% hydrochloric acid up to the 250 mL mark and thenhomogenized. Finally, this solution is used for ICP measurement.

The invention is further exemplified in the examples below. Thefollowing samples were prepared:

Comparative Example 1

CEX1 having a general formula Li_(3.50)Si_(0.50)P_(0.60)O₄ was preparedby the following steps:

1) Mixing: Li₂CO₃, SiO₂, and (NH₄)₂HPO₄ with total weight of around 6.0g according to the corresponding molar ratio were put on a 250 ml bottlewith 140 ml of deionized water and each 100 g of Y doped ZrO₂ ballshaving 3, 5, and 10 mm diameter. The bottle was rotated in aconventional ball mill equipment with 300 RPM for 24 hours. Thehomogeneously mixed slurry was dried at 90° C. for 12 hours.

2) Calcination: the dried mixture was calcined at 900° C. for 6 hours inAr atmosphere.

3) Pulverization: 1.4 g of calcined powder was put on a 45 ml bottlewith 30 ml of acetone and 3.4 g of Y doped ZrO₂ balls having a dimeterof 1 mm. The bottle was rotated in a conventional ball mill equipmentwith 500 RPM for 6 hours. The pulverized powder was dried at 70° C. for6 hours.

4) Sintering: The dried powder was sintered at 700° C. for 3 hours inoxygen atmosphere to get the final LSPO solid electrolyte compound.

Example 1

Ge doped LSPO samples having a general formulaLi_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O₄ were prepared in the samemanner as CEX1 except that different amount of GeO₂ was added and theamount of Li₂CO₃, SiO₂, and (NH₄)₂HPO₄ were adjusted in the mixing stepaccording to the target molar ratios. EX1-A, EX1-B, EX1-C, EX1-D, EX1-E,and EX1-F had the x of 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30,respectively.

Comparative Example 2

S doped LSPO samples CEX2-A1 and CEX2-A2 having a general formulaLi_((3.5−30 x))Si_((0.5−x))P_((0.5−x))S_(2x)O_(4+a) were prepared in thesame manner as samples in the Example 1 except that different amount ofLi₂SO₄ was added instead of GeO₂. CEX2-A1 and CEX2-A2 had the x of 0.05and 0.10, respectively.

Ga doped LSPO sample CEX2-B having a general formulaLi_((3.5−30 x))Si_((0.5−x))P_((0.5−x))Ga_(2x)O_(4+a) was prepared in thesame manner as samples in the Example 1 except that Ga₂O₃ was addedinstead of GeO₂. CEX2-B had the x of 0.05.

Explanatory Example 1

To examine the stability of electrolyte with Li metal, LSPGO samples ofEX1-B is directly contacted with molten Li metal under Ar atmosphere. Tothis end, pellet of EX1-B is made by pressing 0.175 g of EX1-A powderunder 2349.7 kgf/cm² pressure, followed by a sintering at 700° C. for 3h under 02 atmosphere. A stripe of Li foil is placed on astainless-steel plate heated on 250° C. hot plate (under controllednon-oxidizing atmosphere). The obtained molten Li is directly poured onthe prepared EX1-B pellets and the pellets are observed for 15 minutes.This explanatory experiment is conducted in a glove box with Aratmosphere. The pellet is visually observed/monitored so as to detect aneventual thermal runaway and the structure is examined using X-RayDiffraction.

Results

TABLE 1 List of Examples and Comparative Examples with their propertiesSample Conductivity Volume Examples ID Composition x - (S/cm) (As)Comparative CEX1 Li_(3.50)Si_(0.50)P_(0.50)O₄ 0.00 4.4 × 10⁻⁶ 324.15example 1 Example 1 EX1-A Li_(3.55)Si_(0.45)P_(0.45)Ge_(0.10)O₄ 0.05 7.2× 10⁻⁶ 327.11 EX1-B Li_(3.60)Si_(0.40)P_(0.40)Ge_(0.20)O₄ 0.10 1.0 ×10⁻⁵ 329.96 EX1-C Li_(3.65)Si_(0.35)P_(0.35)Ge_(0.30)O₄ 0.15 1.4 × 10⁻⁵332.93 EX1-D Li_(3.70)Si_(0.30)P_(0.30)Ge_(0.40)O₄ 0.20 1.5 × 10⁻⁵335.73 EX1-E Li_(3.75)Si_(0.25)P_(0.25)Ge_(0.50)O₄ 0.25 2.7 × 10⁻⁵338.83 EX1-F Li_(3.75)Si_(0.25)P_(0.25)Ge_(0.50)O₄ 0.30 2.4 × 10⁻⁵340.76 Comparative CEX2- Li_(3.35)Si_(0.45)P_(0.45)S_(0.10)O₄ 0.05 2.3 ×10⁻⁶ 324.27 A1 example 2 CEX2- Li_(3.20)Si_(0.40)P_(0.40)S_(0.20)O₄ 0.103.9 × 10⁻⁷ 323.58 A2 CEX2-B Li_(3.65)Si_(0.45)P_(0.45)S_(0.10)O₄ 0.053.5 × 10⁻⁶ No solid solution (*x in CEX1 and EX1-A to EX1-F with formulaLi_((3.5x))Si_((0.5-x))P_((0.5-x))Ge_(2×04), in CEX2-A1 and CEX2-A2 withformula Li_((3.5-3x))Si_((0.5-x))P_((0.5-x))S_(2x)O₄, and in CEX2-B withformula Li_((3.5+303x))Si_((0.5-x))P_((0.5-x))Ga_(2x)O₄.)

Enhanced Conductivity of the LSPGO According to the Present Invention

Table 1 summarizes the list of example and comparative example samples.It can be seen that LSPGO samples at x=0.05-0.30 (EX1-A to EX1-F) havehigher lithium ionic conductivity than the LSPO sample (CEX1). As for Sand Ga bearing LSPO (CEX2-A1, CEX2-A2, and CEX2-B), the lithium ionicconductivities are lower.

FIG. 1 shows a typical Nyquist plot from EIS measurementLi_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O_(4+a) (x=0.00 to 0.30). Theplot is composed of real part of impedance in X axis and imaginary partin Y axis. High frequency measurement appears on the left side of thegraph forms a semicircle that is followed by a low angle spike as theresult of the low frequency measurement. The semicircle formationcorresponded to the lithium conduction in SE. The lithium ionicconductivity of CEX1 is 4.4×10⁻⁶ at room temperature that is comparablewith the earlier published works on LSPO. The reduction of thesemicircle diameter with the higher Ge amount indicating lowerresistance of sample suggesting a better conductivity.

FIG. 2 shows the improvement of lithium ionic conductivity where thevalue increases proportionally with more Ge in the structure. Thehighest lithium ionic conductivity is 2.7×10⁻⁵ at x=0.25 which is about6 times of CEX1. When x is 0.30, the value reduces to 2.4×10⁻⁵,concluding that 0.25 is an optimal concentration.

Retention of the Solid Solution in the Ge-Doped LSPO According to theInvention

FIG. 3 shows the XRD patterns of CEX1 (undoped electrolyte) and EX1-A toEX1-F. It is observed that Ge bearing LSPO samples have a single LISICONphase without impurity, demonstrating that Ge is well doped. In allgraphs, the γ-Li₃PO₄ structure is maintained, indicated by thecharacteristic double peak at 28°-29° representing (121) and (200)planes as well as four consecutive peaks at 34°-36° correspond to (220),(131), (211), and (002) planes. The shifted peak positions to the leftrelated with the larger crystal volume showed in Table 1. The observedsingle phase also acts as an evidence of solid solution formation forthe Ge bearing LSPO.

Additionally, FTIR measurement is displayed in FIG. 4 with the vibrationbands of P—O, Si—O, and Ge—O marked. The vibration bands at 580 cm⁻¹,910 cm⁻¹, and around 1050 cm⁻¹ correspond to the vibration modes oftetrahedron P—O while band in area of 985-1005 cm⁻¹ corresponds to thevibration modes of Si—O, also in tetrahedral structure. All bands showtransmittance signal increase along with the addition of Ge in thestructure. On the other hand, transmittance of 790 cm⁻¹ band whichcorresponds to Ge—O vibration decreases with the more Ge in thestructure. It is concluded that the atomic substitution of Gesuccessfully occurred at Si and P sites without changing the structure.This observation matches with that of XRD where the structure of LSPO ismaintained upon doping.

XRD of CEX2 are compared with CEX1 in FIG. 5. The S bearing LSPOstructure (CEX2-A) is still well maintained while Ga bearing LSPOstructure (CEX2-B) has multiple impurity peaks marked with (*) in thegraph. This indicating Ga is not easy to be doped in the LSPO system andsolid solution is failed to form for CEX2-B.

Chemical Stability of the LSPGO with a Li Metal Foil

Chemical stability of LSPGO with Li metal was examined through a directcontact with the molten Li metal.

LSPGO pellet XRD diffraction pattern after exposure with molten Li metalis compared with the original pattern before exposure as displayed inFIG. 6. Both measurement produced diffractogram with the same peaklocation indicating the structure remain the same (there is nostructural change as the result of reaction with Li metal). The pelletis also observed to be very stable upon contact without thermal runawayor any exothermic reaction.

Example 2 (Catholyte)

A M-NMC622 compound having the target formula ofLi(Ni_(0.60)Mn_(0.20)Co_(0.20))O₂ and a monolithic morphology isobtained through a double sintering process and a wet milling processrunning as follows:

1) Co-precipitation: mixed transition metal hydroxides with D50 ofaround 4 μm are prepared by the process described in KR101547972B1 (frompage 6 line 25 to page 7 line 32).

2) 1^(st) blending: to obtain a lithium deficient sintered precursor,Li₂CO₃ and the co-precipitation product are homogenously blended with aLi/M′ ratio of 0.85 in a Henschel mixer for 30 minutes so as to obtain a1^(st) blend.

3) 1^(st) sintering: the 1^(st) blend is sintered at 935° C. for 10hours under an oxygen containing atmosphere. The product obtained fromthis step is a powderous lithium deficient sintered precursor withLi/M′=0.85.

4) Blending: the lithium deficient sintered precursor is blended withLiOH.H₂O to correct the Li stoichiometry to Li/M′=1.01. The blending isperformed in a mixer for 30 minutes so as to obtain a 2^(nd) blend.

5) 2^(nd) sintering: the 2^(nd) blend is sintered at 890° C. for 10hours in an oxygen containing atmosphere in a roller hearth kiln (RHK).The sintered blocks are crushed by a jaw crushing equipment.

6) Wet milling: To break the agglomerated intermediate particles intomonolithic primary particles, a wet ball milling process is applied. 5 Lbottle is filled with 1 L of deionized water, 5.4 kg ZrO₂ balls, and 1kg of 2^(nd) sintering product from process number 5. The bottle isrotated on a commercial ball mill equipment.

7) Healing firing step (3rd sintering): The wet milled product is heatedat 750° C. for 10 hours under oxygen containing atmosphere in a furnace.The sintered compound is sieved.

A catholyte material EX2-CAT-A is made by mixing M-NMC622 and EX1-B(Li_(3.60)Si_(0.40)P_(0.40)Ge_(0.20)O₄) according to a mixing ratio of1:1 by weight, followed by a heat treatment at 700° C. for 3 hours in anoxygen atmosphere (i.e. like air). EX1-B has a median particle size(D50) of 2 μm.

A catholyte material EX2-CAT-B is obtained through a similar manner asthe preparation of EX2-CAT-A except that the mixture is heated at 600°C.

Comparative Example 3 (Catholyte)

CEX3-CAT-A and CEX3-CAT-B are obtained through a similar manner as thepreparation of EX2-CAT-A except that the mixture is heated at 500° C.,900° C., respectively, as displayed in Table 2.

TABLE 2 Particle size distribution and ionic conductivity of EX2 andCEX3 material Co-sintering Ionic First discharge Sample temperature D50D99 conductivity capacity ID* (° C.) (μm) (μm) (S/cm) (mAh/g) EX2-CAT-A700 10.2 34.7 1 × 10−5 176.4 EX2-CAT-B 600 10.3 31.0 2 × 10−6 _**CEX3-CAT-A 500 10.4 28.1 1 × 10−7 _** CEX3-CAT-B 900 10.5 143.0 9 × 10−4133.8 *-CAT- stands for “catholyte”. ** - : not measured.

The results of the size distribution measurement as displayed in theTable 2 show that EX2-CAT-A, EX2-CAT-B, CEX3-CAT-A, and CEX3-CAT-B havea similar D50 at around 10.2-10.5 μm. However, the D99 values are largerat the higher co-sintering temperature. For instance, the D99 ofEX2-CAT-A (resulting from a sintering at 700° C.) is 34.7 μm while theD99 of CEX3-CAT-B (resulting from a co-sintering at 900° C.) is 143.0μm. The coin cell characterization of EX2-CAT-A and CEX3-CAT-B asdisplayed in FIG. 7 shows the effect of the co-sintering temperature tothe electrochemical performance. Here, CEX3-CAT-B sintered at 900° C.clearly shows a lower discharge capacity comparing to EX2-CAT-A sinteredat 700° C.

On the other hand, co-sintering temperature lower than 600° C. is alsounpreferable. The ionic conductivity data provided in Table 2 alsodemonstrate that the ionic conductivity of the catholyte depends uponthe co-sintering temperature, wherein the catholyte is more conductiveat a higher co-sintering temperature. EX2-CAT-A which results from aco-sintering prepared at 700° C. has an ionic conductivity of 10⁻⁵ S/cmwhile CEX3-CAT-A (resulting from a co-sintering at 500° C.) has an ionicconductivity of 10⁻⁷ S/cm, i.e. a decrease of ×100 is observed for thissample with respect to CEX3-CAT-A's ionic conductivity.

1-12. (canceled)
 13. A solid solution electrolyte suitable forsolid-state rechargeable lithium ion battery comprising a compoundhaving a general formulaLi_((3.5+L+x))Si_((0.5+s−x))P_((0.5+p−x))Ge_(2x)O_(4+a) wherein−0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.0<x≤0.30.14. The solid solution electrolyte according to claim 13 wherein saidcompound has a general formula:Li_((3.5+x))Si_((0.5−x))P_((0.5−x))Ge_(2x)O₄.
 15. The solid solutionelectrolyte according to claim 13, having a lithium ionic conductivityof at least 10⁻⁵ S/cm at room temperature.
 16. The solid solutionelectrolyte according to claim 13, wherein 0.15≤x≤0.30.
 17. The solidsolution electrolyte according to claim 13, having a lithium ionicconductivity at room temperature superior or equal to 2.0×10⁻⁵ S/cm andinferior or equal to 3.0×10⁻⁵ S/cm.
 18. The solid solution electrolyteaccording to claim 13, comprising a crystal structure having an XRDpattern measured at a wavelength of 1.5418 Å comprising a first peakhaving a first intensity and a second peak having a second intensity,said first and second peaks being present in a range of 2θ superior orequal to 27.5 and inferior or equal to 30.0±0.5°, said XRD pattern beingfurthermore free of peaks at 37.0±0.5°≤2θ≤47.0±0.5° having an intensitysuperior to said first or second intensity.
 19. The solid solutionelectrolyte according to claim 13, comprising a crystal structure havingan XRD pattern measured at a wavelength of 1.5418 Å comprising a firstpeak having a first intensity and a second peak having a secondintensity, said first and second peaks being present in a first range of2θ superior or equal to 27.5 and inferior or equal to 30.0±0.5°, whereinin a second range of 2θ superior or equal to 21.0 and inferior or equalto 25.0±0.5°, said XRD pattern has no more than three peaks, each ofsaid three peaks having an intensity superior to said first or secondintensity.
 20. The solid solution electrolyte according to claim 13,comprising a crystal structure having an XRD pattern measured at awavelength of 1.5418 Å comprising a first peak having a first intensityand a second peak having a second intensity, said first and second peaksbeing present in a first range of 2θ superior or equal to 27.5 andinferior or equal to 30.0±0.5°, wherein in a second range of 2θ superioror equal to 34.0 and inferior or equal to 36.0±0.5°, said XRD patternhas no more than three peaks, each of said three peaks having anintensity superior to said first or second intensity.
 21. A solid-staterechargeable lithium ion battery comprising the solid solutionelectrolyte according to claim
 13. 22. A solid-state rechargeablelithium ion battery comprising a negative electrode having a Limetal-base anode contacting the solid solution electrolyte according toclaim
 13. 23. The solid-state battery according to claim 21 wherein thebattery is configured to have an operating voltage superior or equal to200 V and inferior or equal to 500 V such that the battery is suitablefor use in an electric car.
 24. A catholyte comprising the solidsolution electrolyte according claim 13, and a cathode material havingthe general formula: Li_(1+k)M′_(i−k)O₂ whereM′=Ni_(1−x′−y′−z′)Mn_(x′)Co_(y′)Å_(z′) with −0.05≤k≤0.05, 0≤x′≤0.40,0.05≤y′≤0.40, and 0≤z′≤0.05, wherein A is a doping element which isdifferent to Li, M′ and O, said positive active material powdercomprising particles having a layered R-3m crystal structure, saidcatholyte having a D99≤50 μm and an ionic conductivity of at least1.0×10⁻⁶ S/m.